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Zener clamping output of regulator

Started by panfilero September 11, 2013
Hello,

If I had a little isolated 3.3V voltage regulator like this guy

http://www.recom-power.com/pdf/Econoline/RM.pdf

and I put a 3.3V zener right across the output to clamp the output to 3.3V 

like this zener

http://www.diodes.com/datasheets/ds30410.pdf

but I saw 3.6V across my zener.... any thoughts on what is going on? is it not cool to put a zener at the output to clamp the voltage of the regulator?

much thanks!
On 11/09/2013 15:02, panfilero wrote:
> Hello, > > If I had a little isolated 3.3V voltage regulator like this guy > > http://www.recom-power.com/pdf/Econoline/RM.pdf > > and I put a 3.3V zener right across the output to clamp the output to 3.3V > > like this zener > > http://www.diodes.com/datasheets/ds30410.pdf > > but I saw 3.6V across my zener.... any thoughts on what is going on? is it not cool to put a zener at the output to clamp the voltage of the regulator?
The small print says it doesn't regulate to within its specified tolerances on loads less than 10% and the 3.3v part can be 20% high. If it is absolutely critical that parts never see a voltage above 3.3v even at very low current draw then you need a better regulator. A series resistor between its raw output and the zener may help. -- Regards, Martin Brown
On Wednesday, September 11, 2013 10:02:14 AM UTC-4, panfilero wrote:
> Hello, >=20 >=20 >=20 > If I had a little isolated 3.3V voltage regulator like this guy >=20 >=20 >=20 > http://www.recom-power.com/pdf/Econoline/RM.pdf >=20 >=20 >=20 > and I put a 3.3V zener right across the output to clamp the output to 3.3=
V=20
> like this zener=20 > http://www.diodes.com/datasheets/ds30410.pdf >=20 > but I saw 3.6V across my zener.... any thoughts on what is going on? is i=
t not cool to put a zener at the output to clamp the voltage of the regulat= or?
>=20
Totally *not* cool. Zeners need to operate at a certain current..(well at = least over a small range.) I assume there too much current flowing through= the zener and this puts it above the voltage 'spec'. Low voltage zeners h= ave a very soft knee... look at the curve in the spec sheet you posted. (a= nd observe it's logarithmic in current.) George H.
>=20 > much thanks!
George,

Sure zeners need a certain current, this little regulator can ouput a max o=
f 75mA, if it started to output over 3.3V I would assume the zener would tu=
rn on and start sinking some of that current and clamp at 3.3V, but why wou=
ld it let the voltage across the zener go higher? 75mA is not enough to mak=
e the zener go to 3.6V, I'm thinking the zener isn't turning on at all... b=
ut I don't see why not.


On Wednesday, September 11, 2013 9:17:33 AM UTC-5, George Herold wrote:
> On Wednesday, September 11, 2013 10:02:14 AM UTC-4, panfilero wrote: >=20 > > Hello, >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > If I had a little isolated 3.3V voltage regulator like this guy >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > http://www.recom-power.com/pdf/Econoline/RM.pdf >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > and I put a 3.3V zener right across the output to clamp the output to 3=
.3V=20
>=20 > > like this zener=20 >=20 > > http://www.diodes.com/datasheets/ds30410.pdf >=20 > >=20 >=20 > > but I saw 3.6V across my zener.... any thoughts on what is going on? is=
it not cool to put a zener at the output to clamp the voltage of the regul= ator?
>=20 > >=20 >=20 > Totally *not* cool. Zeners need to operate at a certain current..(well a=
t least over a small range.) I assume there too much current flowing throu= gh the zener and this puts it above the voltage 'spec'. Low voltage zeners= have a very soft knee... look at the curve in the spec sheet you posted. = (and observe it's logarithmic in current.)
>=20 >=20 >=20 > George H. >=20 > >=20 >=20 > > much thanks!
On Wednesday, September 11, 2013 10:12:54 AM UTC-4, Martin Brown wrote:
> On 11/09/2013 15:02, panfilero wrote: > > > Hello, > > If I had a little isolated 3.3V voltage regulator like this guy > > http://www.recom-power.com/pdf/Econoline/RM.pdf > > and I put a 3.3V zener right across the output to clamp the output to 3.3V > > like this zener > > http://www.diodes.com/datasheets/ds30410.pdf > > > > > > but I saw 3.6V across my zener.... any thoughts on what is going on? is it not cool to put a zener at the output to clamp the voltage of the regulator? > > > The small print says it doesn't regulate to within its specified > tolerances on loads less than 10% and the 3.3v part can be 20% high.
Oh I didn't see that.. then maybe just replace the zener with a suitable resistor to ground. (to get some minimum current draw from the regulator.) George H.
> > > If it is absolutely critical that parts never see a voltage above 3.3v > > even at very low current draw then you need a better regulator. A series > > resistor between its raw output and the zener may help. > > > > -- > > Regards, > > Martin Brown
On Wed, 11 Sep 2013 07:02:14 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>Hello, > >If I had a little isolated 3.3V voltage regulator like this guy > >http://www.recom-powefr.com/pdf/Econoline/RM.pdf
Those things are not regulators at ALL. They're kind of the opposite of a regulator, in fact.
>and I put a 3.3V zener right across the output to clamp the output to 3.3V > >like this zener > >http://www.diodes.com/datasheets/ds30410.pdf > >but I saw 3.6V across my zener.... any thoughts on what is going on? is it not cool to put a zener at the output to clamp the voltage of the regulator? > >much thanks!
No, it's not cool. If you need a regulated output, you can use a 5V module and regulate it down to 3.3V.
On Wednesday, September 11, 2013 10:26:43 AM UTC-4, panfilero wrote:
> George, >=20 >=20 >=20 > Sure zeners need a certain current, this little regulator can ouput a max=
of 75mA, if it started to output over 3.3V I would assume the zener would = turn on and start sinking some of that current and clamp at 3.3V, but why w= ould it let the voltage across the zener go higher? 75mA is not enough to m= ake the zener go to 3.6V, I'm thinking the zener isn't turning on at all...= but I don't see why not. Well try making an I-V plot of just the zener. That might help you. =20 Go from maybe 1uA to 10mA. (or add a 300 ohm resistor to ground) George H.=20 <snip of the other double spaced stuff>
On Wednesday, September 11, 2013 11:24:52 AM UTC-4, Spehro Pefhany wrote:
> On Wed, 11 Sep 2013 07:02:14 -0700 (PDT), panfilero >=20 > <panfilero@gmail.com> wrote: >=20 >=20 >=20 > >Hello, >=20 > > >=20 > >If I had a little isolated 3.3V voltage regulator like this guy >=20 > > >=20 > >http://www.recom-powefr.com/pdf/Econoline/RM.pdf >=20 >=20 >=20 > Those things are not regulators at ALL. They're kind of the opposite >=20 > of a regulator, in fact.=20
I've never seen such garbage.
>=20 >=20 >=20 > >and I put a 3.3V zener right across the output to clamp the output to 3.=
3V=20
>=20 > > >=20 > >like this zener >=20 > > >=20 > >http://www.diodes.com/datasheets/ds30410.pdf >=20 > > >=20 > >but I saw 3.6V across my zener.... any thoughts on what is going on? is =
it not cool to put a zener at the output to clamp the voltage of the regula= tor?
>=20 > > >=20 > >much thanks! >=20 >=20 >=20 > No, it's not cool. If you need a regulated output, you can use a 5V >=20 > module and regulate it down to 3.3V.
It's cool for those so-called zeners. Check out the spec sheet, IZT is 50uA= ! Then further down the graphs for zener impedance vs current, it is almost= 10K ohms at that current! It would only take a 1mA or so to elevate the 3.= 3V to 3.6V. One thing for sure, absolutely WRONG part to use as a clamp.
Excellent, I feel like this is the answer but I'm having a little trouble f=
ollowing it

So, 50uA is the test current used to get the 3.3V nominal value right?  I t=
hought you would usually run a zener with a few milliamps to get it to clam=
p...=20

How does this 10k impedance come into play? If it's 3.3V at 50uA, and then =
it gets another 1mA running through it, doesn't the impedance just come dow=
n since there's more current?  I think the answer is here but I just don't =
see it

thanks!

On Wednesday, September 11, 2013 10:26:50 AM UTC-5, bloggs.fred...@gmail.co=
m wrote:
> On Wednesday, September 11, 2013 11:24:52 AM UTC-4, Spehro Pefhany wrote: >=20 > > On Wed, 11 Sep 2013 07:02:14 -0700 (PDT), panfilero >=20 > >=20 >=20 > > <panfilero@gmail.com> wrote: >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > >Hello, >=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >If I had a little isolated 3.3V voltage regulator like this guy >=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >http://www.recom-powefr.com/pdf/Econoline/RM.pdf >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > Those things are not regulators at ALL. They're kind of the opposite >=20 > >=20 >=20 > > of a regulator, in fact.=20 >=20 >=20 >=20 > I've never seen such garbage. >=20 >=20 >=20 >=20 >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > >and I put a 3.3V zener right across the output to clamp the output to =
3.3V=20
>=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >like this zener >=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >http://www.diodes.com/datasheets/ds30410.pdf >=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >but I saw 3.6V across my zener.... any thoughts on what is going on? i=
s it not cool to put a zener at the output to clamp the voltage of the regu= lator?
>=20 > >=20 >=20 > > > >=20 > >=20 >=20 > > >much thanks! >=20 > >=20 >=20 > >=20 >=20 > >=20 >=20 > > No, it's not cool. If you need a regulated output, you can use a 5V >=20 > >=20 >=20 > > module and regulate it down to 3.3V. >=20 >=20 >=20 > It's cool for those so-called zeners. Check out the spec sheet, IZT is 50=
uA! Then further down the graphs for zener impedance vs current, it is almo= st 10K ohms at that current! It would only take a 1mA or so to elevate the = 3.3V to 3.6V. One thing for sure, absolutely WRONG part to use as a clamp.
That's an unregulated converter (and unlimited; basically a two-transistor 
chopper, transformer and rectifier).  As others have said, if you need 
better regulation, do something else.  For example, use the 5V model 
(assuming you also have a 5V input somewhere), followed by an LDO.

Zeners are rather poor accuracy, particularly low voltage devices (under 
7V), which have large dynamic resistances.

If you also need insurance that voltage does not exceed 3.3V plus change 
(3.60, say), use a TL431 to trigger an SCR.  You will also need a current 
limit; an LDO's built in current limit is very crude, and the isolation 
module will smoke rapidly if overloaded (the regulated flyback type 
isolators are current-limited, but not available in small packages).  At 
this point, you might be better off building your own isolator or 
regulator.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

"panfilero" <panfilero@gmail.com> wrote in message 
news:8ca28e60-6b89-4fc9-9b2d-5add67574641@googlegroups.com...
> Hello, > > If I had a little isolated 3.3V voltage regulator like this guy > > http://www.recom-power.com/pdf/Econoline/RM.pdf > > and I put a 3.3V zener right across the output to clamp the output to > 3.3V > > like this zener > > http://www.diodes.com/datasheets/ds30410.pdf > > but I saw 3.6V across my zener.... any thoughts on what is going on? is > it not cool to put a zener at the output to clamp the voltage of the > regulator? > > much thanks!